//云
var cloudObj = function(){
    this.ctx = ctx;
    this.x = [];
    this.y = [];
    this.r = [];
    this.speed = [];
};
cloudObj.prototype.num = 5;
cloudObj.prototype.init = function(){
    for(var i = 0; i < this.num; i++){
        this.x[i] = utils.getRandom(0, canWidth);
        this.y[i] = utils.getRandom(-10, canHeight/5);
        this.r[i] = utils.getRandom(10, 100);
        this.speed[i] = 0.1*Math.random() + 0.1;
    }
};
cloudObj.prototype.draw = function(){
    for(var i = 0; i < this.num; i++){
        this.x[i] += this.speed[i];
        if(this.x[i] > canWidth){
            this.x[i] = -200;
        }
        utils.drawCloud(this.ctx, this.x[i], this.y[i], this.r[i]);
    }
};